NARRATOR: Although “weather” is a description of the state of the atmosphere at a specific time and can include references like sunny, partly cloudy, or drizzly, the types of weather people tend to care most about are severe storms. So, what causes a calm day to suddenly turn violent, or even deadly, and why do some storms live on for hours or even days?
To understand storms you have to understand clouds, which form when warm air rises from Earth’s surface toward the higher and colder reaches of the atmosphere. This upward motion can result when a colder, denser air mass pushes under a warmer one and forces it upward, or when a patch of ground absorbs more heat than its surroundings and transfers that heat to the air above, causing it to rise.
Severe storms like thunderstorms, blizzards, and hurricanes require three conditions to develop and maintain their potentially devastating power: an energy source, moisture, and unstable air.
The energy for big storms can come from temperature differences between two air masses or, more often, from heat absorbed by Earth’s surface that warms the air above it. It’s the same process that creates clouds but on a larger and more extreme scale. The warmer the ground or ocean water below a developing storm, the more energy it can transfer to the storm as it grows.
Severe storms also require huge amounts of moisture. Not only is this moisture necessary to produce a storm’s precipitation, but it also carries the storm's energy from Earth’s surface to the upper reaches of the atmosphere.
The energy is transported by water vapor, which is produced when light from the Sun heats up liquid on Earth’s surface, causing it to evaporate. The vapor holds onto that energy until it condenses and transitions back into the liquid water droplets or ice crystals that make up a cloud. When that happens, the vapor releases the stored energy in the form of heat, which fuels the storm’s growth.
The final critical factor in the formation of severe storms is for the air surrounding the storm to be significantly colder than air at lower levels. Scientists call this "unstable air." So, as a storm cloud rises and releases heat, it continues to be surrounded by colder air, which allows it to rise even further. Basically, the greater the temperature difference between a storm cloud and its surroundings, the faster, higher, and more intense it will grow.
When all of these conditions are in place—a rich source of energy, lots of moisture, and unstable air above—severe storms are sure to develop. Just how powerful a particular storm will be depends on how extreme those conditions are—and they can be very powerful.
The most severe thunderstorms can drop more than two inches of rain per hour and generate winds of more than 200 miles per hour. When multiple thunderstorms form near each other in an area over warm ocean water, they can combine to form the start of a hurricane—Earth’s most powerful and devastating type of storm.